In many known positive displacement meters the typical method of measuring the volume of fluid passing through the meter is by measuring the angular displacement of the meter. As the various mechanical mechanisms in such a meter wear over time, the length of the stroke of the pistons within the meter changes, causing inaccuracies if the angular displacement method of measurement is used. To correct for such inaccuracies as the meter ages, the meter must be periodically recalibrated to correct for increasing wear of the mechanical parts.
The present inventors recognize that in applications where a positive displacement meter is employed for fluid metering, long-term accuracy relative to prior metering systems would be enhanced by measuring the total linear distance traveled by the positive displacement device (the pistons, for example), and using such measurements to calculate the metered volume from the displacement and associated area of the displaceable device. As indicated, the displaceable device could be a piston within a cylinder, or a plurality of pistons within cylinders of a typical positive displacement meter. The accuracy of such a positive displacement meter is dependent upon the resolution of the linear measuring device. As discussed below, there are many prior systems for providing measurement of the linear displacement of a moving element.
Beck et al., U.S. Pat. No. 3,237,150, teaches the use of two transducers in an ultrasonic position indicator system for determining the position of control rods in a nuclear reactor. One transducer is used to ping a fixed target to provide a calibration signal, whereas the other transducer is used to ping a movable target. Acoustic pulses returned from the fixed target are used to provide error compensation for pulses received from the movable target, for accurately determining the position of the nuclear control rod being monitored.
Massa, U.S. Pat. No. 4,210,969, discloses an acoustic ranging system for determining the height of a liquid in a container. The system includes a reflecting target located a predetermined distance from a transducer for providing calibration signals. A microprocessor is included and programmed to provide appropriate timing for the operation of the transducer to transmit acoustical pulses to the fixed target and the surface of the liquid, in such a manner as to prevent "ghost" or false signals.
Ruter, et al., U.S. Pat. No. 4,542,652, teaches a method and apparatus for determining the location of a piston within a cylinder from the uppermost end of the cylinder. A target reflector is used in the cylinder to provide calibration signals and an accordion-like reflector attached to the top face of the piston is used to reflect measuring signals back to the transducer.
Head et al., U.S. Pat. No. 4,543,649, teaches the use of ultrasonics for detecting the position of a piston within a cylinder. In FIG. 12 of this patent a target reflector is located at a fixed position to obtain compensation signals, for correcting errors due to variations in temperature and pressure.
U.K. Patent No. 1,525,720 teaches an acoustical distance measuring system for transmitting an acoustical pulse to a piston, measuring the time for a signal to reflect back to a transducer, and computing from this measured time the distance of the piston from the transducer.
There are many other patents that disclose some form of measurement of a moving object or element. A number of such patents are of interest for background information, and include Erdman, U.S. Pat. No. 2,743,429; Williams, U.S. Pat. No. 2,985,018; Pedersen, U.S. Pat. No. 4,008,455; Newman, U.S. Pat. No. 4,415,914; Rosie et al., U.S. Pat. No. 4,229,798; Beroev, U.S. Pat. No. 4,228,530; Newman, U.S. Pat. No. 4,254,482; Soltz, U.S. Pat. No. 4,470,299; Yamaquchi, U.S. Pat. No. 4,606,015; and Japanese No. 55-152475.